It is known to stretch a synthetic-resin web in its plane either in one direction--so-called uniaxial orientation--or in two transverse directions--so-called biaxial orientation--so as to change important characteristics, such as mechanical strength, of the web. This process is typically done immediately as the web issues from an extruder, while it is still hot, and the stretched web is fed immediately to a thermoforming machine. Since an extruder operates continuously and a thermoformer discontinuously, a speed compensator must be employed so that the web is received continuously but fed out discontinuously.
Such an apparatus is known from German patent document 2,832,385 published in Germany without claim to foreign priority Feb. 14, 1980 based on an application filed July 24, 1978 by A. Thiel. In this apparatus a polystyrene web is first longitudinally stretched between a plurality of rollers, then transversely stretched between a pair of conveyor elements gripped to its longitudinal edges, and finally passed through a speed compensator to the thermoformer. The speed compensator is a roller or the like that is displaceable tranversely of the transport plane of the web so that when it moves away from this plane with a speed equal to half of the transport speed the web stops downstream of it, and, when it moves back toward the plane, the web moves faster downstream than upstream. Such a machine is relatively massive, and the drives of all the various sprockets and wheels and of the speed compensator must all be quite heavily dimensioned and perfectly synchronized to move the web through smoothly. It is normal that the distance between the upstream end of this device, which is formed by the output of the extruder, and the downstream end, which is constituted by the upstream end of the thermoforming mold, is equal to about five mold lengths. The normal travel time through this apparatus is therefore about 10 to 15 seconds.
Study has shown, however, that the molecular orientation or so-called plastic memory that is produced by stretching in a synthetic-resin web, and that so advantageously influences the characteristics of resins like polystyrene, is intimately related to the temperature at which this orientation is set in the resin. Thus the time between when the resin is stretched, which is normally done with the web hot, and the time when the resin cools sufficiently to set the stretch orientation in it is critical. The web relaxes, that is its oriented molecules largely reorient themselves randomly, if the resin stays hot for some time after it is stretched even though the web is held at the enlarged size that was imparted to it during stretching. For example it has been found that polystyrene that is stretched while near its plastification temperature loses approximately half of its orientation within one second.
Thus the above-described prior-art machine is deficient in that it allows an excessive amount of time to elapse after the hot plastics web is stretched. In fact so much time is frequently lost that by the time the stretched resin has reached the thermoformer it is not substantially stronger than it would have been if not stretched.
Attempts to speed up this process, as by discontinuous operation of the stretcher such as described in U.S. Pat. No. 3,172,159, have not worked as such discontinuous operation causes waves or other imperfections in the web.
Another solution is stretching at lower temperatures. Such a procedure is substantially more difficult, however, due to the greater mechanical strength of the web at a lower temperature. In addition it is simply impossible to set the desired molecular orientation in a resin that is not quite hot.